There are many applications, industrial, scientific and medical, in which it is necessary to determine the quantitative levels of particular components or details of a moving system, wherein the component or detail to be measured is situated in a background environment which may be visually difficult to differentiate from the component or detail to be measured. In such cases, conventional imaging methods are not always adequate.
One such example is in the determination of the oxygen level in the blood supply to a living tissue, or of any other recognizable component of the blood supply. Adequate oxygen supply by the blood to the tissue is a fundamental prerequisite for its correct function. Oxygen supply, however, is often impaired as a result of several acute and/or chronic diseases, such as those involving local changes in blood vessels caused by mechanical obstruction or inflammatory processes. Such changes can result, for instance, as an outcome of arteriosclerosis or diabetes, which can cause damage to the tissue at the systemic level and/or can cause well defined pathologies in specific organs, including the heart, brain, eyes, and others. In particular, diseases involving or resulting from decreased oxygen supply by the retinal vasculature, are one of the leading causes of blindness worldwide. Many of these diseases are both progressive and treatable. Thus, early detection is highly desirable because it may lead to preventive treatment.
In the eye, for example, diagnoses are often made on the basis of structural changes that occur in the retina as a consequence of, or together with problems with the retinal oxygen supply. Such structural changes include the consequences of ischemic events, sometimes necessitating the performance of fluorescent angiographies in order for them to be detected, neovascularization, which is the growth of new blood vessels in an attempt to compensate for a reduction in oxygen supply from pre-existing vessels, cotton-wool patches, which are regions in which nerve fiber axoplasmic transport has failed, and even the degeneration of retinal nerve fibers. Once observed, these and other phenomena may be used to diagnose retinal vascular disease, and to begin treatment to ameliorate further degeneration. But these structural changes are indicative of significant irreversible damage which has already occurred. It is therefore, clearly desirable to detect disease earlier, before structural damage occurs. In many cases, parts of the retina that are suffering damage have an impaired oxygen supply or metabolism, and thus might be capable of identification by local abnormalities in the oxygen saturation of capillary blood. Similarly, properly functioning or particularly active retinal regions could be identified by the local oxygen saturation characteristics of their capillary blood. Together, such information about damaged and intact retinal areas could provide important landmarks for limiting as much as possible the damage to healthy tissue resulting from targeted retinal treatments. This information can be divided into two categories: that pertaining to the blood oxygen saturation level in blood vessels, this requiring a knowledge of the spectral composition of the components of the blood flow; and that pertaining to structural changes in the blood vessel geometry itself whether due to the generation of new blood vessels, such as in neovascularization, or due to the apparent disappearance of blood vessels due to blockage of the flow therethrough. Each of these categories will now be dealt with successively.
Methods for measuring blood oxygen saturation should be rapid, quantitative, objective, and as non-invasive as possible. A number of methods exist in the prior art:
Blood gas analysis provides a method of measuring oxygen saturation in blood with high accuracy. It is, however, invasive, since it requires a blood sample from the point of interest and thus, in many cases, cannot be used. Also, the measurement takes time and cannot be performed continuously. In addition, only arterial or venous oxygenation can generally be measured, or, by making a small cut in the tissue under examination, the oxygenation of a mixture of arteriolar, venular and capillary blood.
Pulse oximetry, on the other hand, is non-invasive, and allows continuous measurement. Pulse oximetry exploits the pulsatile nature of blood supply due to the heartbeat. This introduces heart-rate correlated changes in the concentration of hemoglobin in the perfused tissue. These changes in the concentration in turn cause heart-rate correlated changes in light absorption of the tissue, as opposed to the more constant background absorption of the surrounding tissue. Pulse oximetry, however, cannot be applied to blood vessels or blood vessel irrigated areas where, due to the viscous properties of the blood and the elastic properties of the blood vessel system the heartbeat signal has decayed below the detectability threshold. This occurs in capillaries and post-capillary vessels, and in a large part of the retinal vasculature in general. Thus, pulse oximetry, since it relies on arterial pulsation, can generally be used only to provide information on the oxygenation of arterial blood, and not for other vascular components, and in particular, not for capillaries, venules or small diameter veins.
Many methods for the assessment of the oxygenation of a blood sample rely on spectral analysis, exploiting the different absorption spectra of oxy-hemoglobin (HbO2) and deoxy-hemoglobin (Hbr). Each spectrum is distinct, and therefore, in theory, spectral measurements of a sample in a cuvette at only a few wavelengths can, subject to some assumptions, provide information about the amount of each chromophore. Oxygen saturation, in turn, is related to the ratio of oxy-hemoglobin to deoxy-hemoglobin. The value of oxygen saturation, SO2, can be calculated from the equation SO2=[HbO2]/{[HbO2]+[Hbr]}.
In vivo measurements, on the other hand, are more difficult. The main difficulty with in vivo spectrometry methods is posed by the presence of pigments other than oxy- and deoxy-hemoglobin. In the spectral range of interest, the absorption spectra of those pigments, along with those of oxy- and deoxy-hemoglobin, are far from flat, and the portion of the overall spectra due to such pigments is not readily determined in vivo. Furthermore, in spectral measurements relying on reflected light, light intensity is affected not only by chromophores but also by other reflecting entities. Thus, a spectral decomposition of the absolute reflection spectrum is often highly problematic, especially, for instance, in a location such as the retina, where many pigments are involved. Furthermore, reflections from the retina may originate from many sources, and the spectral content of the reflected light is thus affected by chromophores or pigments throughout the surrounding tissue, and not only locally.
Another common disadvantage of all of the above techniques for in vivo oxygen saturation measurement is their intrinsically low spatial resolution, generally allowing the assessment only of systemic blood oxygenation values. None of these techniques allows in vivo visualization of the oxygen saturation in distinct vessels, in particular not at the level of the capillary network and not in a comparative way across the different vascular compartments. Since oxygenation may be different in different capillaries, or as a function of time or of manipulations of the physiological activity, important diagnostic information may be obtained by the use of data sets having image character rather than discrete point-like measurements.
In the present example of retinal diseases, the importance of a more direct method of measuring retinal blood oxygenation is evident from the current interest in fields such as the therapeutic effects of hyperoxia in the case of retinal detachment, described in a publication by R. A. Linsenmeier and L. Padnick-Silver entitled “Metabolic dependence of photoreceptors on the choroids in the normal and detached retina” in Investigative Ophthalmology and Visual Science, Vol. 41(10), pp. 3117-3123 (September 2000), and in the retinal hypoxia characteristic of the early stage of diabetes, before clinically evident retinopathy appears, as described in a publication entitled “Retinal Hypoxia in long-term diabetic cats” by R. A. Linsenmeier et al. published in Investigative Ophthalmology and Visual Science, Vol. 39(9), pp. 1647-1657 (August 1998), and as illustrated by the efforts invested in developing such techniques. A method describing direct oxygen tension measurements performed with a retinal oximeter is published in Diabetes Technol. Ther. Vol. 2(1), pp. 111-3 (Spring 2000). Those measurements were, however, confined to large vessels next to the optical disk in a swine animal model.
There is thus a need for a new method that can measure blood oxygen saturation quantitatively, and which overcomes the presence of other absorbing chromophores or reflecting objects in the tissue. There is also a need for methods that are not single point measurements but offer high resolution images of the values of oxygen saturation and other related parameters in the entire imaged tissue rather than at one point. Such images should preferably be obtained from all vascular types, including capillaries, venules and veins.
In some types of chronic progressive disease involving the vasculature, the decision to begin treatment is directly predicated on the onset of structural changes, which appear to mark a critical point in the disease's progress. Neovascularization in the eye is a structural change that indicates the development of an ocular disease state, which carries a high risk of causing permanent and irreversible damage to the eyesight of a patient Numerous factors are predisposing to neovascularization, prominently including diabetic retinopathy, age-related macular degeneration (AMD) and retinal vascular occlusion. These factors indicate that a patient should be monitored closely for further signs of disease, but by themselves are not enough to begin treatments which themselves may have serious consequences for an individual's sight. Thus, sensitive early detection of the onset of neovascularization is desirable for patients known to be at risk.
Ocular neovascular disease is associated with, and thought to be in part caused by, a deficit in oxygen transport to a region of tissue. Other proposed mechanisms of neovascularization do not necessarily pass through a stage of oxygen deficit. Causes that increase the concentration of angiogenesis factors (such as certain tumors), or that decrease the concentration of vasoinhibitory factors (such as vitrectomy or lensectomy) in the eye may also lead to an increased risk of neovascular disease.
Once begun, neovascularization may progress until it itself becomes a cause of further ocular degeneration through one or more of several mechanisms. By blocking fluid outflow through the trabecular meshwork, neovascularization can contribute directly to the tissue-damaging rise in intra-ocular pressure associated with neovascular glaucoma New vessels are weaker than normal vessels, and prone to hemorrhages that can block sight and reduce blood supply. Hemorrhaging may in turn promote retinal detachment, that leads directly to loss of sight Thus, neovascularization occupies a critical point in the progression of retinal disease, as is more fully described in “Textbook of Glaucoma”, by M. Bruce Shields, M.D., published by Lippincott Williams and Wilkins (Philadelphia), 1997.
Not only is it central to the overall disease process, but neovascular disease is also, as mentioned above, treatable. Currently, the most common intervention in the case of a patient who has developed neovascularization of the eye is panretinal photocoagulation (PRP). This technique, though it usually saves the long-term vision of the patient, is partially destructive to existing visual acuity, and is attended by the risk of complications. It is of benefit, therefore, to apply this treatment only in patients where the risk of further disease progression is highest.
For example, PRP treatment of patients with non-proliferative diabetic retinopathy (NPDR) provides measurable, but moderate long-term protective benefit compared to treating patients whose NPDR has already progressed into the more dangerous proliferative diabetic retinopathy (PDR). At the same time, early PRP treatment exposes a number of patients to disadvantage and risk, even though they would not in fact have developed PDR. Refining clinicians' ability to decide which patients should or should not be treated with PRP would thus be of major practical benefit.
By definition, it is the onset of neovascularization that marks the dividing line between NPDR and PDR—the “proliferative” these two terms contain refers to the proliferation of new blood vessels in the eye. Thus, a better method of detecting and measuring neovascularization would serve to aid clinicians in determining which populations of patients should be treated quickly, and those whose diabetic retinopathy is stable, and does not require immediate intervention. A similar argument applies to the treatment of neovascular disease due to other causes, and in other organs besides the eye, such as vascular occlusion, AMD, and tumor-stimulated neovascularization.
Two primary techniques are currently used to diagnose neovascularization in the eye, flourescein angiography and slit lamp examination. Neovascularization of the eye is often noted first in the iris, though it may be seen also in the retina at the same time. The most sensitive of the two examination techniques, fluorescein angiography, detects peripupillary or retinal leakage from newly grown vessels; however, it is an invasive technique that carries a risk of complications. Furthermore, it is often not available to the primary care physicians on whom many patients at risk rely. When neovascularization is sufficiently progressed, slitlamp examination can also directly visualize abnormal new blood vessel growth. However, this visualization is not as sensitive as fluorescein angiography, and again, requires a physician trained to evaluate the findings.
Neovascularization thus occupies a key role in ophthalmic and other diseases, such as cancer, and in governing decisions about treating such diseases. Existing techniques for evaluating neovascularization suffer from the drawbacks of invasiveness, or of insensitivity, and require specially trained medical personnel and/or hospital facilities. There is a need, therefore, for a means of detecting neovascularization which is non-invasive, sensitive, simple to operate, and gives results which may be easily interpreted by the clinician.
Any system or method for the detection of neovascularization by detecting the generation of new blood vessels, should also be useful for the detection of the blockage of existing blood vessels, by the apparent disappearance of such vessels in successive imaging sessions. Such a phenomenon can result as a side-effect of increased intra-ocular pressure, or as a result of sickle-cell anemia.
The disclosures of all publications mentioned in this specification, are hereby incorporated by reference, each in its entirety.